Thermoplastic powder or slurries, known as dry or liquid plastisols, are conventionally cast on the inner surface of a thin metal mold member heated to the fusion temperature of the deposited material to form a gel coat which is heated sufficiently to form a thin, solid thermoplastic molded member (i.e., a shell or skin). The thin metal mold member remains heated until a thin layer of thermoplastic material is built up on the inner surface to form a thin plastic shell product, such as a product suitable for use as an outer skin or shell covering on automotive interior or exterior products such as door panels, instrument panels, bumper moldings or the like.
The method and apparatus for the production of molded skins or shells are well known, such methods often being referred to as "rotational" or "slush" molding. The mold cavity defined by the mold shell is filled with a flowable plastic material and, due to heating of the mold shell, a thin layer of plastic material adheres to the surface of the mold shell and the remaining plastic material can then be shaken or poured out of the mold, as by rotating the mold. When the mold shell is further heated, the skin or shell product clinging thereto gels out. The finish skin or shell product can then be removed after the mold has cooled down. The operation of introducing the plastic material, when in the form of a powder or granule, is typically effected by rotating the mold so that its mold cavity opens downwardly, and then fixing onto the mold a box which contains the plastic material. The overall assembly consisting of the mold and box is then rotated through about 180.degree. so that the plastic material drops into the mold. After the plastic skin or shell has set against the heated mold surface, the mold assembly is rotated back to its original position and the box and excess powder removed, with further heating of the mold then effecting proper melting and gelling of all of the plastic material adhered to the mold surface. The mold is then cooled and the molded skin or shell product removed.
In a typical rotational molding apparatus of the type briefly described above, the mold assembly includes a generally hollow mold housing or box on which the metal mold shell is mounted, which housing and mold shell cooperate to define a hollow interior chamber to which heated air is supplied so as to effect heating of the mold shell from the back surface thereof. This heating of the mold shell, however, has long presented a technical problem since the irregularity in the shape of the mold shell and the length thereof have often made it difficult to provide the desired degree of heat uniformity as applied to the mold shell throughout the back surface area thereof. The mold shell thus often has undesired hot or cold spots which effect the quality (i.e., both thickness and finished surface properties) of the molded skin product. In an attempt to provide better control over the heat applied to the back surface of the mold shell, various complex flow control arrangements have been proposed, including providing the chamber rearward of the mold shell with a plate having numerous pipes or nozzles thereon for directing flow of heated air to specific areas or regions defined on the back surface of the mold shell. Such arrangements, however, are structurally complex and expensive, and in particular require that a different nozzle or pipe arrangement be designed for cooperation with each mold shell.
The prior arrangements also typically perform the entire molding sequence at a single working station, such as by first preheating the mold, then attaching the mold powder box to deposit the powder in the mold, then further heating the mold to effect complete gelling of the powder material to define the molded skin product, and thereafter cooling the mold and effecting removal of the skin product, prior to initiating a new cycle. Due to the requisite time required to properly effect both heating and cooling, the overall product forming cycle is often undesirably long, and thus the single station mold lacks desired productivity and efficiency.
It is an object of this invention to provide an improved molding apparatus and method, such as a slush or rotational molding apparatus and method, for forming a thin skin or shell product, which improved method and apparatus is believed to significantly improve the overall performance characteristics of the apparatus and process in terms of efficiency and rate of productivity, and is also believed to provide for more desirable control over the supplying of heat to the back surface of the mold shell by utilizing a simple and readily adjustable flow control arrangement, thereby providing for the necessary control over the heating of the shell to provide a desirable molded skin product.
In the improved process and apparatus of the present invention, the mold assembly includes a hollow box on which a conventional metal mold shell is mounted, which box and shell cooperate to define a hollow chamber which extends longitudinally of the box and is defined adjacent the back surface of the metal mold shell. Heated air is supplied into one end and discharged from the other end of the chamber to effect heating of the metal shell. The chamber contains therein a plurality of flow control devices which are disposed at spaced intervals within and along the chamber and are adjustable and programmable so as to divert the heated air from conventional longitudinal flow into transverse flow either toward or away from the back surface of the metal mold shell. In the preferred embodiment the flow control device includes a rotatable fan wheel which is positioned within the chamber and, when rotated, effects flow of heated air axially thereof in a direction transverse of the chamber. The fan wheel is driven by a reversible variable-speed motor so that the fan wheel can direct heated air either toward or away from the adjacent back surface of the mold shell, and the speed of the fan wheel can be varied so as to also adjust the quantity of heated air which is directed toward or away from the adjacent back surface of the mold shell. The numerous flow control devices have the motors thereof individually controlled by a control arrangement which enables all of the flow control devices to be individually controlled, both with respect to their on-off cycle, and also with respect to flow direction and speed, to thus provide for rather close control over the amount of heated air which is permitted to directly contact the back surface of the metal mold shell so as to provide for desirable control over the heating of the mold shell throughout the entire surface area thereof. With the arrangement of this invention, the individual flow control devices can each be individually programmed in terms of on-off cycle, direction of rotation, and speed of rotation so as to provide for optimum heating of the adjacent back surface of the metal mold shell, and can also be readily adjusted so as to adjust or accommodate metal mold shells of different configurations or shapes.
The improved process and apparatus of this invention also preferably includes a structure employing a movable carriage or shuttle on which a pair of mold assemblies are provided, which shuttle arrangement can be driven back and forth so as to alternately position one of the mold assemblies in alignment with a source of heated air to effect heating of the mold and deposit of plastic powder and gelling of powder onto the mold surface. When the molding of the skin product has been completed, the shuttle is then shifted into an alternate position so that the mold containing the skin product is shifted into a cooling station and subjected to cooling, such as water or air, and the other mold assembly is then in the heating and mold forming station. While the other mold assembly is in the heating and mold forming station, the first mold assembly is being cooled and the molded skin removed, whereby the first mold assembly is now capable of being moved back to the heating station for performing a further molding operation.
The advantageous arrangement and method of the present invention, and the objects and purposes thereof, will be apparent to persons familiar with molding of this general type upon reading the following specification and inspecting the accompanying drawings.